To avoid rebreathing by the patient, a nonreturn valve is inserted into the branch of the respirator that delivers the breathing gas to the patient. Breathing gas flows through this so-called inspiration branch in the rhythm of respiration during the phase of inspiration, and the nonreturn valve is opened during inspiration. The nonreturn valve closes the flow path during the phase of expiration. To determine the volume flows of inspiration and expiration, for example, flow sensors in the form of hot wire anemometers are known from the state of the art, which operate in such a way that the resistance of a hot wire, which is arranged in the breathing gas flow, is determined. The hot wire is made of a material that has a temperature-dependent resistance. The extent to which the hot wire is cooled by the gas flow depends on the volume flow and hence on the velocity of flow, so that the resistance of the hot wire is an indicator of the velocity of flow. One drawback of this process is, however, the fact that these hot wire sensors require a defined flow situation, which requires a certain section for homogenizing the flow and hence a certain overall length.
Both the flow sensors and the nonreturn valves are integrated with additional components, for example, a respiration drive and pressure sensors, in a respirator or anesthesia apparatus in a respiration system. This respiration system is designed such that it can be removed from the anesthesia apparatus for cleaning purposes. Due to the nonreturn valve and the flow sensor being arranged in series, a space requirement that essentially predefines the size of the entire respiration system is obtained in the respiration system. This makes it difficult to construct a compact respiration system.